Loop communication systems are desirable for many terminal applications such as retail, banking, medical and other similar environments. In such an environment, it is important to allow the operator to turn off power at one or more terminals when they are not being used. Normally, when power is turned off at a terminal, relay points at the terminal close to bypass the electrical connection of the loop communication link around the inoperative terminal.
It is also a well known fact that the distance which a signal can be propagated on a communication line is approximately inversely proportional to the square root of the frequency at which the signal is propagated. Therefore, in low speed communication links, where the probabilities are such that an operating terminal will be present every few miles, there is no need to have separate repeaters to power up and reshape the signals propagating on the loop. On the other hand, when the amount of traffic on the loop requires wider band width, the resultant higher frequencies which must be propagated will require reshaping at closer intervals. In some examples of low speed communication links of the prior art, the repeaters used to reshape the propagating signals on the loop are packaged in the base of each terminal and derive their power from the terminal power supplies. In high speed links where repeaters are needed and an operating terminal cannot be guaranteed, the repeaters derive their power from the signal lines themselves, as taught in U.S. Pat. No. 3,876,983.
An important feature of any loop communication system is the ability to locate and circumvent faults along the loop communication path. The prior art teaches various modes of operation, including routing the inbound and outbound loop links through the same physical locations so that portions of the loop may be eliminated as taught in U.S. Pat. No. 3,458,661. Another way of obtaining this feature is to provide a main loop and a standby loop routed through the same physical locations such as taught in U.S. Pat. No. 3,519,750. The prior art teaches the use of complex logic at each repeater which is mounted in the base of each terminal and, therefore, may share such logic with the terminal and be powered from the terminal power supplies for accomplishing the fault location and isolation functions. When the repeaters are to be physically mounted separate from the terminals, it becomes costly to provide such logic in each repeater, both from a monetary and a power consumption point of view. For example, if such logic must be powered via the signal communication lines, only a few repeaters could be driven because the higher current required by the logic will cause voltage drops approaching the voltage limitations on signal communication lines imposed by local electrical codes.
Therefore, a need has existed for some time for a simple, low cost but high speed loop communication link having a large number of repeaters which are capable of performing fault location and isolation functions, yet draw their power from a single master terminal over the loop signal lines.